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Summary Establishment of symbiosis between plants and arbuscular mycorrhizal (AM) fungi depends on fungal chitooligosaccharides (COs) and lipo‐chitooligosaccharides (LCOs). The latter are also produced by nitrogen‐fixing rhizobia to induce nodules on leguminous roots. However, host enzymes regulating structure and levels of these signals remain largely unknown.Here, we analyzed the expression of a β‐N‐acetylhexosaminidase gene ofMedicago truncatula(MtHEXO2) and biochemically characterized the enzyme. Mutant analysis was performed to study the role ofMtHEXO2during symbiosis.We found that expression ofMtHEXO2is associated with AM symbiosis and nodulation.MtHEXO2expression in the rhizodermis was upregulated in response to applied chitotetraose, chitoheptaose, and LCOs.M. truncatulamutants deficient in symbiotic signaling did not show induction ofMtHEXO2. Subcellular localization analysis indicated that MtHEXO2 is an extracellular protein. Biochemical analysis showed that recombinant MtHEXO2 does not cleave LCOs but can degrade COs intoN‐acetylglucosamine (GlcNAc).Hexo2mutants exhibited reduced colonization by AM fungi; however, nodulation was not affected inhexo2mutants.In conclusion, we identified an enzyme, which inactivates COs and promotes the AM symbiosis. We hypothesize that GlcNAc produced by MtHEXO2 may function as a secondary symbiotic signal.
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KIN3 impacts arbuscular mycorrhizal symbiosis and promotes fungal colonisation in Medicago truncatula
Summary Arbuscular mycorrhizal fungi help their host plant in the acquisition of nutrients, and this association is itself impacted by soil nutrient levels. High phosphorus levels inhibit the symbiosis, whereas high nitrogen levels enhance it. The genetic mechanisms regulating the symbiosis in response to soil nutrients are poorly understood. Here, we characterised the symbiotic phenotypes in fourMedicago truncatula Tnt1‐insertion mutants affected in arbuscular mycorrhizal colonisation. We located theirTnt1insertions and identified alleles for two genes known to be involved in mycorrhization,RAM1andKIN3. We compared the effects of thekin3‐2andram1‐4mutations on gene expression, revealing that the two genes alter the expression of overlapping but not identical gene sets, suggesting thatRAM1acts upstream ofKIN3.Additionally,KIN3appears to be involved in the suppression of plant defences in response to the fungal symbiont.KIN3is located on the endoplasmic reticulum of arbuscule‐containing cortical cells, andkin3‐2mutants plants hosted significantly fewer arbuscules than the wild type.KIN3plays an essential role in the symbiotic response to soil nitrogen levels, as, contrary to wild‐type plants, thekin3‐2mutant did not exhibit increased root colonisation under high nitrogen.
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- Award ID(s):
- 2139351
- PAR ID:
- 10367802
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- The Plant Journal
- Volume:
- 110
- Issue:
- 2
- ISSN:
- 0960-7412
- Page Range / eLocation ID:
- p. 513-528
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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